I. Field of the Invention
This invention relates generally to optical detection systems, and more particularly to optical detection systems capable of detecting laser radiation.
II. Discussion of the Prior Art
The laser is finding its way into the modern world in applications ranging from military applications to surveyor's transits at construction sites. Accompanying the proliferation of lasers is an awareness that lasers can be dangerous, unseen threats to people and things. Many common lasers emit their radiation at wavelengths outside of the range of human sight, and are, therefore, invisible. They are no less dangerous for all their invisibility, however, and some method of knowing whether or not persons or objects are being exposed to laser radiation could be effective in preventing damage caused by laser radiation.
Prior art devices such as those disclosed in U.S. Pat. No. 3,801,825 to Schwartz, et al. use a plurality of at least two nonlinear limiting detectors to detect coherent radiation produced by a device such as a laser. The nonlinear limiting detectors must be selected from a group of materials having the property of absorbing input energy at a fundamental frequency into energy states other than at the fundamental. The energy at the fundamental is limited through a nonlinear process of harmonic generation and absorption and multiple photon absorption to an absolute limiting value. A lens focuses high intensity radiation on a first limiter. The outputs of the first limiter and a second limiter are processed by differencing with the effect that incoherent radiation is cancelled out in the output of a signal processor. Any difference beyond a preselected threshold value is indicative of coherent radiation. The system does not use interference fringes as part of its detection mechanism.
U.S. Pat. No. 4,158,506 to Collett teaches an electro-optical system for determining the polarization of optical pulses. The system comprises a six element optical polarizer and detector assembly with a one-quarter wavelength optical plate positioned over two of the polarizers. The outputs of the detectors are supplied to sum and difference amplifiers to determine the stokes parameters of an optical pulse incident to the system. No method of differentiating coherent radiation from incoherent radiation is disclosed.
This disclosure describes an optical technique for establishing when a person or object is under laser illumination. The sensor described herein could be employed to automatically shut-down the laser, alert a person to danger or initiate a battlefield countermeasure.
Laser light possesses a number of unique characteristics which not only make it useful, but also provide characteristics by which it may be easily identified. Laser illumination is monochromatic, or of a single wavelength. By knowing the wavelength, a filtered detector receiving a large stimulus will signal the probable presence of laser illumination. There are, however, non-laser sources which emit heavily in the same wavelengths as most lasers, so the "color" discrimination of laser light is unreliable.
Laser light is able to travel great distances in an intense, collimated beam. Collimation is the condition which exists when all the "rays" of light projected by a source travel in a parallel bundle. Incoherent light may be collimated, as in the case of a movie projector, but not to the extent that a laser's radiation is collimated. Laser light is collimated because it is coherent, and it is this coherence that above all else distinguishes the laser from other light sources.
A light source is said to be coherent when all points lying on an optical wavefront propagating forth from the source are in phase with one another. A point source emits coherent light, for each point on the expanding spherical wavefront is fully in phase with each other point, and the wavefront will reach some distance from the source simultaneously at all places. Most light sources are not coherent, or are "incoherent", because many point sources, each emitting light at its own random time, produce the net illumination. The wavefronts emitted from such a source have no constant phase relationships. When a thermal light source (incoherent) is collimated by a projection lens, the light leaving the lens is said to be "partially coherent", for some order has been introduced into the previously random propagation. Because no laser is ideal, all lasers are in fact emitting only partially coherent light, the coherence determined by the distance one may travel along the wavefront before one point bears no constant phase relationship to the point at the other end of the trip. FIG. 1 demonstrates this relationship. Points P1 and P2 are located on a coherent wavefront. Regardless of the distance from the emitting source, E, the points have a constant phase relationship.
Optical interference is the term applied to the phenomena whereby one optical wavefront disturbs the passage of another. When the two wavefronts possess some fixed phase relationship over the entire region of interaction, the disturbance is uniform over the entire region, and what is essentially a microscopic phenomena takes on macroscopic (visible) effects. The illumination of a surface with two interfering wavefronts results not in a uniform illumination, but instead in a periodic pattern of bright and dark regions, called "fringes". FIG. 2 shows a plot, S, of light intensity within a fringe pattern. It represents a cyclic pattern of bright and dark regions. Because a laser's radiation is coherent, it can be made to interfere with itself. Incoherent light will not (at least on a macroscopic level). The present invention described takes advantage of this interference phenomena to identify the presence of laser illumination.